This invention relates to transmission networks and, more particularly, to bidirectional voice frequency repeaters.
In telephone and similar communication systems it is often required to enhance signals being transmitted over a 2-wire bidirectional transmission path. Additionally, it is often required to couple a 2-wire bidirectional transmission path to a 4-wire transmission path including an incoming path and an outgoing path. Signal enhancement in a bidirectional 2-wire transmission path typically is realized by employing hybrid circuits to couple signals on the 2-wire path to separate unidirectional paths each including unidirectional amplifiers. Similar arrangements are employed for 2-to-4 wire and 4-to-2 wire transmission path coupling except that only one hybrid circuit is required. Heretofore, special transformers and precision balancing networks have been used in hybrid arrangements. Use of such transformers and balancing networks is undesirable because of cost, size and difficulty in adjusting the balance circuit to match the impedance of the incoming lines. Recently, automatic balancing networks have been developed for this purpose. However, their complexity and cost have limited their use.
So-called electronic hybrid arrangements have been proposed for coupling a 2-wire bidirectional transmission path to a 4-wire path including two unidirectional paths which may or may not include amplification for enhancing signal transmission on the bidirectional path. In one such electronic hybrid first and second transformers are employed to couple unidirectional amplification stages into the bidirectional 2-wire transmission path. Signals from a first direction are supplied from a first 2-wire path via the first transformer to a first one of the unidirectional amplification stages and, then, via the second transformer to a second 2-wire path. Similarly, signals from a second direction are supplied from a second 2-wire path via the second transformer to a second one of the unidirectional amplification stages and, then, via the first transformer to the first 2-wire path, thereby providing amplification for both directions of transmission on the bidirectional 2-wire transmission path.
One problem with this prior known arrangement is that a portion of the signal being transmitted in each direction on the 2-wire path appears at and is amplified by the amplification stage for the opposite direction of transmission. That is to say, there is some cross coupling of signals being transmitted in each direction resulting in error signals. Consequently, the outputs from the amplification stages are distorted by the so-called error signals. Several so-called hybridless bidirectional repeaters have been proposed which attempt to minimize the effects of these error signals. In one particular circuit, a portion of the signal developed at the output of each unidirectional amplificiation stage is supplied to an input of the other unidirectional amplification stage where it is combined with the input from the associated transformer in an attempt to cancel the error signal developed across the transformer associated with the amplification stage. This is achieved by employing resistive elements to couple a portion of each amplifier output to an input of the other amplifier. One problem with this approach is that the use of resistive elements does not provide compensation for the complex impedance presented by most transmission paths, for example, 2-wire telephone lines or the like, or for the complex impedance presented by the coupling transformers. Consequently, the prior known circuits are, at best, limited to applications involving connection to purely resistive impedances as distinguished from complex impedances presented by real telephone lines. Indeed, this prior hybridless bidirectional repeater circuit is known to suffer from frequency instability and when employed to realize practical gain lvels with real telephone lines may oscillate, cause echo signals, or both.
These problems of the prior known arrangement may be partially mitigated by employing a line build-out network in an attempt to make the transmission path appear to be purely resistive. Use of a line build-out network is undesirable because of cost and difficulty of adjustment. Additionally, interaction between gain and impedance adjustments in such prior arrangements further complicates adjustment procedures. Moreover, the line buildout network still would not compensate for the complex impedance of the coupling transformer. Indeed, if a line build-out network is employed in the transmission path, additional distortion components are, therefore, added to the signals being transmitted which would necessarily require additional equalization, again increasing complexity and cost of the bidirectional hybridless repeater.